Project Summary Increases in life expectancy in almost all childhood-onset chronic conditions has brought unique challenges for adolescents and young adults (AYA) who struggle to deal with the associated disease burden, manage therapies, and thrive as they develop independent self-management skills, and become active and engaged patients. The challenges that influence the lives of AYAs are largely adaptive, such as making lifestyle modifications, adhering to complex medication regimens, and learning to navigate the adult health system. Adding to this complexity is the need for AYAs to progressively take over greater self-management responsibilities from parents. Promoting activated self-management is critically important given that this shift in health care management from parents and health care providers to the AYA is identified as key to successful adult outcomes. Peer support interventions are well-suited to address challenges theorized as critical to AYAs given the importance of peer relationships during this time. We propose a mixed-methods, five-year randomized controlled trial, that will include 225 AYAs (16-22 years) with COCCs, to test the Peer i-Coaching for Activated Self- Management Optimization (PICASO) versus an attention control group. This novel, mobile health intervention utilizes an established telephone/text based secure interface to allow AYAs access knowledge, experience, and instrumental/emotional support from a trained peer coach (18-26 years), who has already developed independence and is an active self-manager. We will determine the efficacy of the PICASO on self-management/health care navigation, patient activation, coping, emotional health and health-related quality of life across 12 months. We will explore whether age, sex, race/ethnicity, chronic condition, and/or disease severity moderate the trajectory of PICASO effects on self-management, patient activation, coping, emotional health and health-related quality of life. Lastly, we will explore mechanisms of the PICASO impact by describing AYA experiences with the intervention.